Every unit in the military has a unit historian. Start with his unit's name, and you should be able to pull up unit histories on the web, or (at least) via a hard-copy from the Service branch's Office of the Historian (or some such title). They are very accomodating to serious requests. If they aren't, request it through your Member of Congress.
Also, check the civilan papers published under the "Army Times," "Navy Times," etc. banner. These usually have scores of listings for unit reunions in the back. Such reunions are run by the actual vets themselves and are a good source for contacts. The Army/Navy etc Times are the best of a number of such publications.
There are also opportunites to pay for ads saying: "anyone ever heard of my uncle joe, who was with the 8th Bomb Group in Europe, 1944-45?" Again, the key point is to have as much unit information as possible. These are known as "lost buddies" or something like that. I believe these run in the Army/Air Force/Navy/Marine Times as well.
There are also a number of web sites for such things, but I haven't looked at them in years.